In recent years, in order to perform power supply or perform charging, wireless power transfer techniques have been gaining attention. Research and development are being conducted regarding a wireless power transfer system wirelessly performing power transfer to various electronic apparatuses such as mobile terminals and notebook computers and household electrical appliances or to power infrastructure equipment.
When wireless power transfer is used, standardization is preferably performed so that power sources which transmit power and power receivers which receive the power transmitted from the power sources are used without trouble even when they are products manufactured by different manufacturers.
Conventionally, techniques using electromagnetic induction, and techniques using radio waves are generally known as wireless power transfer techniques.
Recently, wireless power transfer techniques using strong coupling resonance have been attracting attention as techniques being capable of transferring power to a plurality of power receivers while placing each power receiver at a certain distance from a power source, or to various three-dimensional positions of each power receiver.
Wireless power transfer techniques using magnetic field resonance or electric field resonance, for example, are known as this kind of wireless power transfer using strong coupling resonance.
Conventionally, in order to perform power supply or perform charging, wireless power transfer techniques for wirelessly transferring power have been gaining attention, as described earlier. A wireless power transfer system which employs such a wireless power transfer technique normally transfers power to a plurality of power receivers and may involve power transfer control based on, for example, the power required by each power receiver or the positional relationship of each power receiver relative to the power source.
In recent years, power supply control which uses kQ values in power sources (power source coils) and power receivers is being researched and developed. Specifically, for example, research is being conducted to perform wireless power transfer by switching, based on the kQ values, between a time-division power transfer mode in which power is sequentially transferred to a plurality of power receivers and a simultaneous power transfer mode in which power is simultaneously transferred to a plurality of power receivers.
A research is also being conducted for detuning by grouping a plurality of power receivers or shifting the resonance point of a power receiver (a power receiver resonance coil) on the basis of the kQ values. In the future, a variety of proposals to use kQ values are expected to be made in wireless power transfer systems.
The kQ value (kQ) is the product of the k value (k) indicating the degree of coupling of electromagnetic fields (magnetic fields or electric fields) and the Q value (Q) indicating the degree of loss of an electromagnetic field. The larger the k value, the higher the degree of coupling, and the larger the Q value, the lower the degree of loss.
Unfortunately, in the wireless power transfer system, it is difficult to accurately calculate (obtain) a k value, and accordingly a kQ value with each power receiver and no useful proposals have currently been made.
A variety of wireless power transfer techniques have conventionally been proposed.
Patent Document 1: International Publication No. WO 2009/014125 pamphlet
Patent Document 2: Japanese Laid-open Patent Publication No. 2013-198327